This invention relates generally to cathode ray picture tubes and is specifically addressed to a novel processing means for high-resolution cathode ray tubes having shadow masks of the tensed foil type. The invention has particular utility in the faceplate screening process.
A high-resolution color cathode ray tube that utilizes a tensed foil shadow mask typically includes three electron guns arranged in a delta or in-line configuration. Each gun projects an electron beam through the assigned apertures of a shadow mask, also called a "color selection electrode," onto a target area on the inner surface of the faceplate. The target area comprises a pattern of phosphor deposits arranged in groups of triads of dots. Each of the triads consists of a dot of a red-, green-, and blue-light-emitting phosphor. To improve the brightness of the display and to minimize the incidence of color impurities which can result if a beam falls upon an improper phosphor deposit, the target area may include a layer of a darkish light-absorbing material called a "grille" that surrounds and separates each of the dots. This type of screen is known as a "matrix" or "black surround" screen. Alternately, the phosphor and grille deposits on the target surface may comprise a plurality of vertically oriented, spaced rectangles in coordinate relationship to apertures in the form of rectangles or "slots" in the shadow mask. Tubes of this type are referred to as "slot mask tubes," in contrast to the "dot screen" types of tubes.
The phosphor pattern, whether dots, or stripes, is typically formed by a direct photoprinting process. The target area is first coated with a photosensitive slurry comprising phosphor particles of one of the three phosphors described. The shadow mask, mounted in a frame, is temporarily installed in precise relationship to the faceplate, and the coating is exposed to actinic light projected through the apertures of the mask from a light source located at a position that corresponds to the beam-emission point of the related electron gun. The faceplate is separated from the shadow mask and the coating is "developed" to remove unexposed portions. The result is a pattern of dots or stripes capable of emitting light of one color, whether red, green or blue. The mask is then reregistered with the faceplate, and the steps are repeated for each of the remaining colors to deposit triads of phosphor deposits on the target area on the faceplate in coordinate relationship with each aperture of the mask. A further step, usually taken before the deposition of the phosphors, is the application of the black surround.
The screening process requires a mechanism whereby the faceplate may be removed and replaced in precise registration with the shadow mask for the black surround and each of the three colors. The conventional "domed" shadow mask, which is mounted on a stiff frame, is made repeatably registrable with the phosphor deposits screened on the faceplate by a suspension system comprising three or four leaf springs. The springs are spot welded to the mask frame at selected points around its periphery. The distal ends of the springs are apertured to engage studs which project inwardly from the rearwardly extending flange of the tube faceplate. Demounting the mask is accomplished by depressing the springs to disengage the studs, and separating the panel from the mask frame, usually by automatic machinery. In the remounting, following the deposition of a phosphor, the mask and faceplate are again brought into propinquity whereby the springs are caused to re-engage the studs. This process does not lend itself to the screening of a tube that utilizes a foil mask (which is the subject of this application) because of the lack of structural strength of the very thin foil and the very different structures used to support it within the bulb. Therefore, a different means of mask-faceplate referencing is required for tension-mask tubes.
A major problem in manufacturing a color tube with a foil-type shadow mask is the difficulty in re-aligning the panel with the mask accurately enough to maintain registration between the mask apertures and the associated phosphor deposits. In this type of mask, there is a need for greater precision in registration. Foil mask thickness is typically in the range of 0.0002 to 0.002 mils, and the diameter of the apertures in a dot screen tube is about 0.0035 inch. The "pitch" (distance between aperture centers) for use in a high-resolution display may be, for example, 0.3 millimeters, and for very high resolution tubes, 0.2 millimeters or 0.15 millimeters or less. With regard to the Q-distance, this measurement is a function of the pitch of the shadow mask. For example, for a given gun design, if the pitch is 0.3 millimeters, the Q-distance may be, by way of example, 0.330 inch. For the very high resolution pitch of 0.2 millimeters, the Q-distance may be about 0.210 inch. These values dictate the need for great precision in the registration and re-registration of the mask and faceplate. For example, a tolerance of .+-.0.0006 inch in mask-faceplate registration is acceptable in the manufacture of conventional standard resolution cathode ray tubes having the domed mask. In the manufacture of the high resolution tension mask tubes with which this invention is concerned, however, the allowable tolerance is about .+-.0.0002 inch. If this tolerance is exceeded, color purity can be degraded.